Communication system

ABSTRACT

A gateway is described which facilitates a change of communication cell for a mobile device in a communication system, which includes a core network. The gateway receives messages from a base station operating a cell and forwards the messages received from the base station to the core network. The gateway intercepts a message relating to a change of communication cell, from a source cell in which the mobile communication device is located to a target cell, to determine if the core network needs to be notified of the change of cell. When it is determined that the core network needs to be notified, the gateway generates a message for providing information relating to the change of cell to the core network and transmits the message to the core network.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.14/769,884, filed Aug. 24, 2015, which is based on InternationalApplication No. PCT/JP2014/059910 filed Mar. 28, 2014, which is based onand claims priority from United Kingdom Patent Application No. 1306216.1filed Apr. 5, 2013, the contents of all of which are hereby incorporatedby reference in their entirety.

TECHNICAL FIELD

The present invention relates to a communication system and tocomponents thereof for providing communication services to mobile orfixed communication devices. The invention has particular but notexclusive relevance to the implementation of so-called ‘small’ cells orLow Power Nodes (LPNs) in Long Term Evolution (LTE) communicationsystems currently being developed by the 3^(rd) Generation PartnershipProject (3GPP).

BACKGROUND ART

In 3GPP LTE networks, a base station (i.e. evolved NodeB, eNB) of aRadio Access Network (RAN) transmits data and signalling between a corenetwork (CN) and User Equipment (UEs) located within the base station'scoverage area.

Recent developments in communication networks have seen increaseddeployment of so called ‘small’ cells operated by Low Power Nodes(LPNs), such as pico eNBs, femto cells, Home eNBs (HeNBs) or the like,which cells have a smaller coverage area than existing macro cellsoperated by a higher power macro base station. Networks comprising anumber of different cell types, for example a network comprising a macrocell and a femto cell, are referred to as Heterogeneous Networks, orHetNets.

The LPNs/small cell base stations that operate small cells can typicallycommunicate with the core network and with macro base stations by asmall cell gateway (small cell GW′). A small cell GW typically has socalled home evolved nodeB gateway (HeNB-GW) functionality to provideconnectivity from the LPN/small cell base station, to the core network,and X2-GW functionality effectively to provide X2 interface connectivity(albeit indirect) from the LPN/small cell base station to the macro basestations.

More recently the need to make further enhancements to small cells usinglow-power nodes, has been identified as one of the most important topicsfor further development of 3GPP standards compliant communicationsystems in order to enable such communication systems to cope withincreases in mobile traffic especially for hotspot deployments in indoorand outdoor scenarios. According to this interest in small cellenhancements, scenarios and requirements for small cell enhancementswere studied and captured in a 3GPP technical report (3GPP TR 36.932V12.1.0) the contents of which are herein incorporated by reference. TR36.932 defines a low-power node as generally meaning a node whosetransmit power is lower than that of macro node and base stationclasses. For example, as indicated above, both pico eNB and femto HeNBsare considered to be low power nodes.

Small cell enhancements for E-UTRA and E-UTRAN will focus on additionalfunctionalities for enhanced performance in hotspot areas for indoor andoutdoor using such low power nodes.

Currently, the average geographic density of macro cells is 5-7 macrocells per square kilometre. However, it is predicted that the number ofsmall cells in urban areas will reach 40 small cells per squarekilometre. The increasing geographic density of small cells presentschallenges in achieving successful and efficient handover procedures inHetNets.

R1-130748, the contents of which are herein incorporated by reference,is a 3GPP text proposal for TR36.932, on ‘Small Cell EnhancementScenarios’, which describes a number of scenarios for further evaluationincluding, for example: a scenario in which macro and small cells areprovided on the same carrier frequency (intra-frequency); a scenario inwhich macro and small cells are provided on different carrierfrequencies (inter-frequency); and a scenario in which macro cellcoverage is not present, resulting in the presence of only small cells,on one or more carrier frequency.

In each scenario, the cells are considered to be connected via anon-ideal backhaul (e.g. a typical backhaul widely used in the marketsuch as xDSL, microwave, and other backhauls like relaying) as opposedto via an ideal backhaul (e.g. a very high throughput and very lowlatency backhaul such as dedicated point-to-point connection usingoptical fiber).

3GPP technical document R2-130845, the contents of which are hereinincorporated by reference, comprises the basis for a 3GPP technicalreport (3GPP TR 36.842 V0.1.0) that highlights a number of issues thatneed resolving for each of these scenarios including, inter alia:

-   -   a) mobility robustness;    -   b) difficulties in improving system capacity by utilising radio        resources in more than one base station;    -   c) increased signalling load (e.g., to CN) due to frequent        handover;    -   d) difficulties in improving per-user throughput by utilising        radio resources in more than one base station;    -   e) network planning and configuration effort;

Referring, for example, to issue (c) relating to increased signallingload (e.g., to CN) due to frequent handover, handover between the smallcells may comprise a so called ‘S1’ based handover involving S1signalling with a mobility management entity (MME) in the core network,which MME is responsible for controlling mobility of a UE between cells(e.g. where there is no X2 connectivity between the small cells).Alternatively, handover between the small cells may comprise an ‘X2’based handover involving X2 signalling with the target small cell basestation and which involves S1AP path switch messages being exchanged viathe core network.

In the case of the former, S1 based handover situation, for example, thefollowing messages typically need to be exchanged for inter small cellhandover involving the core network:

-   -   S1: HO Required to MME    -   S1: HO Request to another small cell    -   S1: HO Request Ack from small cell to MME    -   S1: HO Command from MME to small cell    -   Handover Notify    -   UE Context Release Command    -   UE Context Release Complete

And optional messages of:

-   -   eNB Status Transfer    -   MME Status Transfer    -   Location Report

Thus, MME signalling load for every handover includes at least sevenmessages related to the radio access network. Moreover, furthersignalling from a further core network entity (a serving gateway (S-GW))is required to switch the communication tunnel with the core network(GTP tunnel) from source small cell to target small cell.

In the case of the former, X2 based handover situation, for example, MMEsignalling load for every handover comprises two messages and the MMEwill also ensure that the communication tunnel with the core network(GTP tunnel) is switched from the source cell to target cell (requiringanother two messages—namely an S1AP PATH SWITCH REQUEST and an S1AP PATHSWITH REQUEST ACK message).

In the case of the scenario in which macro cell coverage is not present,whilst the absence of a macro cell may reduce interference from themacro on the same carrier as one or more of the small cells, the absenceof a macro cell can also add to the complexity of addressing the issueshighlighted above.

SUMMARY OF INVENTION

Accordingly, preferred exemplary embodiments of the present inventionaim to provide methods and apparatus which overcome or at leastalleviate the above issues.

In one aspect, the invention provides a gateway apparatus forfacilitating a change of communication cell for a mobile communicationdevice in a communication system, the gateway apparatus being operableto receive messages from at least one base station operating arespective cell of said communication system and to forward the messagesreceived from the at least one base station to a communication entity ina core network, the gateway apparatus comprising: receiving means forreceiving, from at least one of a source base station that operates asource cell in which said mobile communication device is located and atarget base station that operates a target cell for said change ofcommunication cell, at least one message relating to said change ofcommunication cell; means for intercepting and processing said messagerelating to said change of communication cell to determine if saidcommunication entity in the core network needs to be notified ofinformation relating to said change of communication cell; generatingmeans for generating, when it is determined that said communicationentity in the core network needs to be notified of information relatingto said change of communication cell, a message for providing saidinformation relating to said change of communication cell to said corenetwork entity; and transmitting means for transmitting, to saidcommunication entity in the core network, said message for providingsaid information relating to said change of communication cell.

The means for intercepting and processing may be operable to determinewhether or not said message relating to said change of communicationcell should be forwarded to said communication entity in the corenetwork.

The means for intercepting and processing may be operable to determinewhether messages relating to a change of communication cell should beforwarded to said communication entity in the core network based on anumber of cell changes that have occurred since messages relating to achange of communication cell were previously forwarded to saidcommunication entity in the core network. In this case, the means forintercepting and processing may be operable to determine that messagesrelating to a change of communication cell should not be forwarded tosaid communication entity in the core network when the number of cellchanges that have occurred since messages relating to a change ofcommunication cell were previously forwarded to said communicationentity in the core network has not exceeded a predetermined value. Themeans for intercepting and processing may also be operable to determinethat messages relating to a change of communication cell should beforwarded to said communication entity in the core network when thenumber of cell changes that have occurred since messages relating to achange of communication cell were previously forwarded to saidcommunication entity in the core network has reached or exceeded thepredetermined value.

The generating means may be operable to generate, responsive to saidmessage relating to said change of communication cell, a further messagefor progressing said change of communication cell; and said transmittingmeans may be operable to transmit said further message for progressingsaid change of communication cell to said source base station or to saidtarget base station. In this case, the generating means may be operableto generate said further message for progressing said change ofcommunication cell without forwarding the message relating to saidchange of communication cell to the communication entity in the corenetwork. The generating means may be operable to generate said furthermessage for progressing said change of communication cell based oninformation obtained by said means for intercepting and processing frommessages previously forwarded to or from said communication entity insaid core network.

The means for intercepting and processing may be operable to determineif said communication entity in the core network needs to be notified ofinformation relating to said change of communication cell comprisinglocation information. In this case, the message for providing saidinformation relating to said change of communication cell to saidcommunication entity in the core network may comprise a location reportfor providing said location information.

The means for intercepting and processing may be operable to determineif said communication entity in the core network needs to be notified ofinformation relating to said change of communication cell comprisinginformation identifying communication bearers that the target basestation has failed to set up. In this case, the message for providingsaid information relating to said change of communication cell to saidcommunication entity in the core network may comprise a messageidentifying said communication bearers that the target base station hasfailed to set up.

The receiving means may be operable to receive, from said communicationentity in the core network in response to said message identifying saidcommunication bearers that the target base station has failed to set up,a message for initiating a modification in a communication parameter forcommunication by said mobile communication device in said target cell.In this case, the message for initiating a modification in acommunication parameter may be configured to initiate a change in anaggregate maximum bit rate.

The means for intercepting and processing may be operable to determine,from a message relating to said change of communication cell receivedfrom said source base station, whether direct data forwarding ispossible for at least one communication bearer of said mobilecommunication device. In this case, the generating means may be operableto generate, responsive to said determination of whether direct dataforwarding is possible, a message indicating whether direct dataforwarding is possible for sending to said target base station.

The generating means may be operable to generate, responsive to adetermination that direct data forwarding is possible, a messagecomprising parameters for establishing a direct communication tunnel forforwarding data directly between said source and target base stations.

The gateway apparatus may be operable, responsive to a determinationthat direct data forwarding is not possible, to facilitate establishmentof an indirect communication tunnel for forwarding data indirectlybetween said source and target base stations via said gateway apparatus.The gateway apparatus may also be operable to facilitate saidestablishment of an indirect communication tunnel using a tunnelcreation request (e.g. a Create Indirect Data Forwarding Tunnel Requestmessage) and a corresponding response message (e.g. a Create IndirectData Forwarding Tunnel Response message).

The gateway apparatus may be operable to terminate messages related tosaid change of communication cell, received from said source basestation or from said target base station and destined for saidcommunication entity in the core network, without forwarding them tosaid communication entity in the core network.

The gateway apparatus may be a small cell gateway apparatus. The gatewayapparatus may comprise at least one of a home base station gateway(HeNB-GW) function, a security gateway (SeGW), and an X2 gateway (X2-GW)function. The communication entity in the core network may be a mobilitymanagement entity (MME). The base station may comprise a low power node(LPN) operating a small cell (e.g. a pico, femto or micro cell). The atleast one message relating to said change of communication cell may be amessage in accordance with the S1 application protocol (S1AP).

In another aspect, the invention provides a gateway apparatus forfacilitating a change of communication cell for a mobile communicationdevice in a communication system, the gateway apparatus comprising:means for storing, at the gateway apparatus, context information for themobile communication device relating to a communications connectionbetween the mobile communication device and a source base station thatoperates a source cell in which said mobile communication device islocated; means for receiving, from at least one of a source base stationthat operates a source cell in which said mobile communication device islocated and a target base station that operates a target cell for saidchange of communication cell, at least one message relating to saidchange of communication cell; means for generating at least one furthermessage for progressing said change of communication cell responsive tosaid at least one message relating to said change of communication cellusing said context information; and means for transmitting, to at leastone of said source base station and said target base station, said atleast one further message for progressing said change of communicationcell.

In this case, the at least one further message for progressing saidchange of communication cell may comprise a message for transferringsaid context information to said target base station. The contextinformation may comprise security context information relating to thecommunications connection between the mobile communication device andthe source base station (e.g. including at least one security key). Thegateway apparatus may be operable to determine whether or not toinitiate the transfer of said context information, including saidsecurity context information, based on a number of cell changes thathave occurred since the security context information was last updated.The gateway apparatus may be operable to determine to initiate thetransfer of said context information, including said security contextinformation, when the number of cell changes that have occurred sincethe security context information was last updated has not exceeded apredetermined value.

The gateway apparatus may be operable to determine not to initiate thetransfer of said context information, including said security contextinformation, when the number of cell changes that have occurred sincethe security context information was last updated has reached orexceeded said predetermined value. In this case, the gateway apparatusmay be operable to initiate an update to said security contextinformation when the number of cell changes that have occurred since thesecurity context information was last updated has reached or exceededsaid predetermined value.

In yet another aspect, the invention provides a gateway apparatus forfacilitating a change of communication cell for a mobile communicationdevice in a communication system, the gateway apparatus being operableto receive messages from at least one base station operating arespective cell of said communication system and to forward the messagesreceived from the at least one base station to a communication entity ina core network, the gateway apparatus comprising: means for receiving,from at least one of a source base station that operates a source cellin which said mobile communication device is located and a target basestation that operates a target cell for said change of communicationcell, at least one message relating to said change of communicationcell; means for intercepting and processing said message relating tosaid change of communication cell to determine whether said messagerelating to a change of communication cell should be forwarded to saidcommunication entity in the core network based on a number of cellchanges that have occurred since messages relating to a change ofcommunication cell were previously forwarded to said communicationentity in the core network; means for generating, when it is determinedthat said message relating to a change of communication cell should notbe forwarded to said communication entity in the core network, a furthermessage for progressing said change of communication cell; means fortransmitting, when it is determined that said message relating to achange of communication cell should not be forwarded to saidcommunication entity in the core network, to said source base station orto said target base station, said further message for progressing saidchange of communication cell; and means for forwarding said messagerelating to a change of communication cell to said communication entityin the core network, when it is determined that said message relating toa change of communication cell should be forwarded to said communicationentity in the core network.

In a further aspect, the invention provides a gateway apparatus forfacilitating a change of communication cell for a mobile communicationdevice in a communication system, the gateway apparatus being operableto receive messages from at least one base station operating arespective cell of said communication system and to forward the messagesreceived from the at least one base station to a communication entity ina core network, the gateway apparatus comprising: means for receiving,from at least one of a source base station that operates a source cellin which said mobile communication device is located and a target basestation that operates a target cell for said change of communicationcell, at least one message relating to said change of communicationcell; means for intercepting and processing said message relating tosaid change of communication cell to determine, from a message relatingto said change of communication cell received from said source basestation, whether direct data forwarding is possible for at least onecommunication bearer of said mobile communication device; means forgenerating, responsive to said determination of whether direct dataforwarding is possible, a message indicating whether direct dataforwarding is possible for sending to said target base station; andmeans for transmitting, said message indicating whether direct dataforwarding is possible to said target base station.

In another aspect, the invention provides a gateway apparatus forfacilitating a change of communication cell for a mobile communicationdevice in a communication system, the gateway apparatus being operableto receive messages from at least one base station operating arespective cell of said communication system and to forward the messagesreceived from the at least one base station to a communication entity ina core network, the gateway apparatus comprising a processor andtransceiver circuitry, wherein: said transceiver circuitry is configuredto receive, from at least one of a source base station that operates asource cell in which said mobile communication device is located and atarget base station that operates a target cell for said change ofcommunication cell, at least one message relating to said change ofcommunication cell; said processor is configured to: i) intercept andprocess said message relating to said change of communication cell todetermine if said core network entity needs to be notified ofinformation relating to said change of communication cell; and ii)generate, when it is determined that said communication entity in thecore network needs to be notified of information relating to said changeof communication cell, a message for providing said information relatingto said change of communication cell to said communication entity in thecore network; and said transceiver circuitry is configured to transmit,to said communication entity in the core network, said message forproviding said information relating to said change of communicationcell.

In another aspect, the invention provides a gateway apparatus forfacilitating a change of communication cell for a mobile communicationdevice in a communication system, the gateway apparatus comprising aprocessor, a memory, and transceiver circuitry, wherein: said processoris configured to store in said memory context information for the mobilecommunication device relating to a communications connection between themobile communication device and a source base station that operates asource cell in which said mobile communication device is located; saidtransceiver circuitry is configured to receive, from at least one of asource base station that operates a source cell in which said mobilecommunication device is located and a target base station that operatesa target cell for said change of communication cell, at least onemessage relating to said change of communication cell; said processor isconfigured to generate at least one further message for progressing saidchange of communication cell responsive to said at least one messagerelating to said change of communication cell using said contextinformation; and said transceiver circuitry is configured to transmit,to at least one of said source base station and said target basestation, said at least one further message for progressing said changeof communication cell.

In another aspect, the invention provides a gateway apparatus forfacilitating a change of communication cell for a mobile communicationdevice in a communication system, the gateway apparatus being operableto receive messages from at least one base station operating arespective cell of said communication system and to forward the messagesreceived from the at least one base station to a communication entity ina core network, the gateway apparatus comprising a processor andtransceiver circuitry, wherein: said transceiver circuitry is configuredto receive, from at least one of a source base station that operates asource cell in which said mobile communication device is located and atarget base station that operates a target cell for said change ofcommunication cell, at least one message relating to said change ofcommunication cell;

said processor is configured to: i) intercept and process said messagerelating to said change of communication cell to determine whether saidmessage relating to a change of communication cell should be forwardedto said communication entity in the core network based on a number ofcell changes that have occurred since messages relating to a change ofcommunication cell were previously forwarded to said communicationentity in the core network; and ii) generate, when it is determined thatsaid message relating to a change of communication cell should not beforwarded to said communication entity in the core network, a furthermessage for progressing said change of communication cell; means fortransmitting, when it is determined that said message relating to achange of communication cell should not be forwarded to saidcommunication entity in the core network, to said source base station orto said target base station, said further message for progressing saidchange of communication cell; and said transceiver circuitry isconfigured to forward said message relating to a change of communicationcell to said communication entity in the core network, when it isdetermined that said message relating to a change of communication cellshould be forwarded to said communication entity in the core network.

In another aspect, the invention provides a gateway apparatus forfacilitating a change of communication cell for a mobile communicationdevice in a communication system, the gateway apparatus being operableto receive messages from at least one base station operating arespective cell of said communication system and to forward the messagesreceived from the at least one base station to a communication entity ina core network, the gateway apparatus comprising a processor andtransceiver circuitry, wherein: said transceiver circuitry is configuredto receive, from at least one of a source base station that operates asource cell in which said mobile communication device is located and atarget base station that operates a target cell for said change ofcommunication cell, at least one message relating to said change ofcommunication cell; said processor is configured to: i) intercept andprocess said message relating to said change of communication cell todetermine, from a message relating to said change of communication cellreceived from said source base station, whether direct data forwardingis possible for at least one communication bearer of said mobilecommunication device; and ii) generate, responsive to said determinationof whether direct data forwarding is possible, a message indicatingwhether direct data forwarding is possible for sending to said targetbase station; and said transceiver circuitry is configured to transmit,said message indicating whether direct data forwarding is possible tosaid target base station.

The invention also provides a communication system comprising the abovedescribed gateway apparatus, at least one mobile communication device, asource base station and a target base station.

In another aspect, the invention provides a method performed by agateway apparatus for facilitating a change of communication cell for amobile communication device in a communication system, the gatewayapparatus being operable to receive messages from at least one basestation operating a respective cell of said communication system and toforward the messages received from the at least one base station to acommunication entity in a core network, the method comprising:receiving, from at least one of a source base station that operates asource cell in which said mobile communication device is located and atarget base station that operates a target cell for said change ofcommunication cell, at least one message relating to said change ofcommunication cell; intercepting and processing said message relating tosaid change of communication cell to determine if said communicationentity in the core network needs to be notified of information relatingto said change of communication cell; generating, when it is determinedthat said communication entity in the core network needs to be notifiedof information relating to said change of communication cell, a messagefor providing said information relating to said change of communicationcell to said communication entity in the core network; and transmitting,to said communication entity in the core network, said message forproviding said information relating to said change of communicationcell.

In another aspect, the invention provides a method performed by agateway apparatus for facilitating a change of communication cell for amobile communication device in a communication system, the methodcomprising: storing, at the gateway apparatus, context information forthe mobile communication device relating to a communications connectionbetween the mobile communication device and a source base station thatoperates a source cell in which said mobile communication device islocated; receiving, from at least one of a source base station thatoperates a source cell in which said mobile communication device islocated and a target base station that operates a target cell for saidchange of communication cell, at least one message relating to saidchange of communication cell; generating at least one further messagefor progressing said change of communication cell responsive to said atleast one message relating to said change of communication cell usingsaid context information; and transmitting, to at least one of saidsource base station and said target base station, said at least onefurther message for progressing said change of communication cell.

In another aspect, the invention provides a method performed by agateway apparatus for facilitating a change of communication cell for amobile communication device in a communication system, the gatewayapparatus being operable to receive messages from at least one basestation operating a respective cell of said communication system and toforward the messages received from the at least one base station to acommunication entity in a core network, the method comprising:receiving, from at least one of a source base station that operates asource cell in which said mobile communication device is located and atarget base station that operates a target cell for said change ofcommunication cell, at least one message relating to said change ofcommunication cell; intercepting and processing said message relating tosaid change of communication cell to determine whether said messagerelating to a change of communication cell should be forwarded to saidcommunication entity in the core network based on a number of cellchanges that have occurred since messages relating to a change ofcommunication cell were previously forwarded to said communicationentity in the core network; generating, when it is determined that saidmessage relating to a change of communication cell should not beforwarded to said communication entity in the core network, a furthermessage for progressing said change of communication cell; transmitting,when it is determined that said message relating to a change ofcommunication cell should not be forwarded to said communication entityin the core network, to said source base station or to said target basestation, said further message for progressing said change ofcommunication cell; and forwarding said message relating to a change ofcommunication cell to said communication entity in the core network,when it is determined that said message relating to a change ofcommunication cell should be forwarded to said communication entity inthe core network.

In another aspect, the invention provides a method performed by agateway apparatus for facilitating a change of communication cell for amobile communication device in a communication system, the gatewayapparatus being operable to receive messages from at least one basestation operating a respective cell of said communication system and toforward the messages received from the at least one base station to acommunication entity in a core network, the method comprising:receiving, from at least one of a source base station that operates asource cell in which said mobile communication device is located and atarget base station that operates a target cell for said change ofcommunication cell, at least one message relating to said change ofcommunication cell; intercepting and processing said message relating tosaid change of communication cell to determine, from a message relatingto said change of communication cell received from said source basestation, whether direct data forwarding is possible for at least onecommunication bearer of said mobile communication device; generating,responsive to said determination of whether direct data forwarding ispossible, a message indicating whether direct data forwarding ispossible for sending to said target base station; and transmitting, saidmessage indicating whether direct data forwarding is possible to saidtarget base station.

Aspects of the invention extend to computer program products such ascomputer readable storage media having instructions stored thereon whichare operable to program a programmable processor to carry out a methodas described in the aspects and possibilities set out above or recitedin the claims and/or to program a suitably adapted computer to providethe apparatus recited in any of the claims.

Although for efficiency of understanding for those of skill in the art,the invention will be described in detail in the context of a 3G system(UMTS, LTE), the principles of the invention can be applied to othersystems (such as WiMAX) in which (home) base stations communicate via asignalling gateway with the corresponding elements of the system changedas required.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 schematically illustrates a mobile telecommunication system of atype to which the invention is applicable;

FIG. 2 schematically illustrates a mobile telecommunication system ofanother type to which the invention is applicable;

FIG. 3 is a block diagram illustrating the main components of a mobilecommunication device forming part of the system shown in FIG. 1 or FIG.2;

FIG. 4 is a block diagram illustrating the main components of a basestation forming part of the system shown in FIG. 1 or FIG. 2;

FIG. 5 is a block diagram illustrating the main components of a smallcell gateway forming part of the system forming part of the system shownin FIG. 1 or FIG. 2;

FIG. 6 is an exemplary timing diagram illustrating a method performed bycomponents of the mobile telecommunication system of FIG. 1 whilstcarrying out an embodiment of the invention;

FIG. 7 is an exemplary timing diagram illustrating another methodperformed by components of the mobile telecommunication system of FIG. 1whilst carrying out an embodiment of the invention;

FIG. 8 is an exemplary timing diagram illustrating another methodperformed by components of the mobile telecommunication system of FIG. 1whilst carrying out an embodiment of the invention;

FIG. 9 is an exemplary timing diagram illustrating another methodperformed by components of the mobile telecommunication system of FIG. 1whilst carrying out an embodiment of the invention;

FIG. 10 illustrates an architecture for a small cell gateway;

FIG. 11 illustrates another architecture for a small cell gateway;

FIG. 12 shows an exemplary control plane architecture for an exemplarycommunication system;

FIG. 13 shows an exemplary user plane architecture for an exemplarycommunication system;

FIG. 14 is an exemplary timing diagram for an initial RRC connectionestablishment by a mobile communication device; and

FIG. 15 is an exemplary timing diagram illustrating a handover scenarioin a communication system configured for virtual dual connectivity.

MODE FOR CARRYING OUT THE INVENTION Overview

FIGS. 1 and 2 each schematically illustrates a mobile (cellular)telecommunication system 1 including a mobile communication device 3comprising a mobile telephone (or other compatible user equipment) and aplurality of small cell base stations 5-1 to 5-3 (which may be anysuitable small cell base station for example, Home evolved NodeBs(HeNBs), pico or femto base stations or the like) but which may be anysuitable small cell base station) each of which operates an associatedsmall cell 10-1 to 10-3 arranged in a cluster.

Each base station 5 is coupled to a core network 7 via a small cellgateway 14 and the core network 7 is also coupled to other networks(e.g. the Internet) via one or more gateways (not shown). The corenetwork 7 includes a mobility management entity (MME) 12, a servinggateway (S-GW) 16 (and other communication entities such as a PacketData Network (PDN) Gateway (PGW), which have also been omitted for sakeof simplicity). The mobile communication device 3 is served via a smallcell 10-2 operated by one of the base stations 5-2.

The backhaul from the base stations 5 to the core network 7 isconsidered to be non-ideal (e.g. comprising a typical backhaul such asxDSL or microwave). Further, in the arrangement of cells shown in FIG. 1there is no macro cell, within the vicinity, which the mobilecommunication device 3 can connect to or can receive information (suchas system information) or reference signals from. Accordingly, thisconforms with the scenario, described in R1-130748, in which macro cellcoverage is not present, resulting in the presence of only small cells,on one or more carrier frequency

Communication between the base stations 5 and the MME 12 is via aso-called ‘S1’ interface. An ‘X2’ interface is also provided forcommunication between neighbouring base stations 5 to facilitate dataexchange between them. As those skilled in the art will appreciate,whilst one mobile communication device 3 and three base stations 5 areshown in FIG. 1 for illustration purposes, additional user equipmentand/or base stations may be present in a deployed system. Asillustrated, the mobile communication device 3 is located within cell10-2, and base station 5-2 acts as a serving cell for the mobilecommunication device 3. Connection of the mobile communication device 3with a serving base station 5-2 allows the mobile communication device 3to perform communication actions such as registering with the network,updating its registration, and establishing a data session or a voicecall.

However, when the mobile communication device 3 approaches the edge ofcell 10-2, or when the signal conditions within this cell begin todeteriorate, for example due to path loss, physical environment or powerlimitation, it becomes necessary for the mobile communication device 3to handover to another cell, such as cell 10-1 or 10-3, or any othersuitable cell.

Further reasons for handover becoming necessary may include, forexample, the presence of regions where the signals from the serving basestation 5-2 cannot be decoded by the mobile communication device 3.Also, in uplink, because a base station may communicate with many mobilecommunication devices, its resources may become limited or insufficient.Such limited cell coverage or lack of base station resources results infewer services for users and on-going communication/applications maysuffer, for example experiencing increased latency.

The serving base station 5-2 receives information from the mobilecommunication device 3 which it uses to determine when handover becomesnecessary for the mobile communication device 3, and to identify aplurality of suitable candidate target cells. The information receivedby the base station 5-2 could be any information measured by the mobilecommunication device 3 or information deduced from the measurementsperformed by the mobile communication device 3.

The mobile communication device 3 is therefore configured (by or viabase station 5-2) to perform signal measurements with respect to itsneighbouring cells and/or serving cell and to report the results basedon these measurements when certain predetermined signal conditions aremet.

The small cell base stations 5 and MME 12 are operable in accordancewith the so-called S1 application protocol (S1AP) in accordance with3GPP TS 36.413 V11.3.0 the contents of which are herein incorporated byreference. S1AP provides the signalling service between the small basestations 5 of the E-UTRAN and the evolved packet core (EPC) that isrequired to fulfil a number of S1AP functions including, inter alia:

-   -   Evolved Radio Access Bearer (E-RAB) management functions for        setting up, modifying and releasing E-RABs, which are triggered        by the MME 12. The release of E-RABs may be triggered by the        base station 5 as well.    -   Initial Context Transfer functions to establish an S1 context        for the mobile communication device 3 in the base station 5, to        setup the default IP connectivity, to setup one or more E-RAB(s)        if requested by the MME 12, and to transfer non-access stratum        (NAS) signalling related information to the base station 5 if        needed.    -   UE Capability Information Indication functions to provide the        capability information for the mobile communication device 3,        when received from the mobile communication device 3 to the MME        12.    -   Mobility Functions for mobile communication devices 3 operating        in an active mode to enable a change of base station via the S1        interface and/or a change of radio access network (RAN) nodes        between different radio access technologies (RATs) via the S1        interface.    -   Paging functionality which allows the EPC the capability to page        the mobile communication device 3.    -   S1 interface management functions including: reset functionality        to ensure a well defined initialisation on the S1 interface;        error Indication functionality to allow a proper error        reporting/handling in cases where no failure messages are        defined; an overload function to indicate the load situation in        the control plane of the S1 interface; a load balancing function        to ensure equally loaded MMES 12 within an MME pool area; S1        setup functionality for initial S1 interface setup for providing        configuration information; base station 5 and MME 12        configuration update functions are to update application level        configuration data needed for the base station 5 and MME 12 to        interoperate correctly on the S1 interface.    -   Non-Access Stratum (NAS) signalling transport functionality        between the mobile communication device 3 and the MME 12: to        transfer NAS signalling related information and to establish the        S1 context for the mobile communication device 3 in the base        station 5; to transfer NAS signalling related information when        the S1 context for the mobile communication device 3 in the base        station 5 is already established.    -   S1 UE context release functionality to manage the release of a        mobile communication device specific context in the base station        5 and the MME 12.    -   UE context modification functionality to modify the established        context for the mobile communication device 3 partly.    -   Status transfer functionality for transferring Packet Data        Convergence Protocol (PDCP) Sequence Number (SN) status        information from a source base station to a target base station        in support of in-sequence delivery of data and duplication        avoidance for intra LTE handover.    -   Location reporting functionality to allow the MME 12 to be aware        of the mobile communication devices current location.

The small cell gateway 14 is provided both: with conventional HeNB-GWfunctionality for receiving and forwarding S1 signalling between thebase station 5 and the MME 12; and with X2-GW functionality forreceiving and forwarding X2 signalling between base stations.

Beneficially, in one exemplary embodiment (as illustrated in FIG. 1), S1mobility related signalling for the purposes of handover is terminatedin the small cell gateway 14 as opposed to the MME 12 as isconventionally the case, thereby avoiding additional signalling with thecore network 7.

Specifically, the small cell gateway 14 is able to receive S1signalling, from the base station 5-2 of a source small cell 10-2, thatindicates that handover is required. The small cell gateway 14 is,advantageously, able to determine, based on this S1 signalling, withoutinvolving the core network 7 (e.g. without involving the MME 12), thathandover is required. Further, the small cell gateway 14 is able togenerate, when it is determined that handover is required, appropriateS1 signalling for communicating with the base stations 5 of a targetcell (e.g. cell 10-1 or cell 10-3) and the source cell 10-2, (and forreceiving associated response signalling from the respective basestations 5 of the target cell (e.g. cell 10-1 or cell 10-3) and thesource cell 10-2) in pursuit of handover of the mobile communicationdevice 3 from the source small cell 5-2, to the target small cell (e.g.cell 10-1 or cell 10-3) without additional, unnecessary signallingdirected to the MME 12.

It is noted, however, that termination of handover signalling in thesmall cell gateway 14 does not, by itself, provide the flexibility tohandle some more complex situations that may become increasinglyprevalent as small cell scenarios become more ubiquitous.

In this exemplary embodiment, greater flexibility is beneficiallyprovided, by allowing some communication between the small cell gateway14 and the core network 7 even where handover is terminated at the smallcell gateway 14.

Advantageously, for example, greater flexibility is beneficiallyprovided by having the small cell gateway 14 monitor whether set up ofany communication bearers supported at the source small cell basestation 5-2 has failed at the target base station 5-1 or 5-3. If set upof any communication bearers supported at the source small cell basestation has failed at the target base station 5-1 or 5-3, then the smallcell gateway 14 communicates with the MME 12, and the target small cellbase station 5-1 or 5-3 in order to initiate an appropriate change inthe aggregate maximum bit rate for the mobile communication device 3.

The aggregate maximum bit rate (UE-AMBR) limits the aggregate bit ratethat can be expected to be provided across all non-guaranteed bit rate(non-GBR) bearers for a mobile communication device. Accordingly,updating the UE-AMBR as described above allows for better utilisation ofthe resources, when communication bearers are rejected, because itallows the UE-AMBR to be dynamically reduced so that resources can beallocated to other mobile communication devices.

Moreover, in this exemplary embodiment, greater flexibility isbeneficially provided by having the small cell gateway 14 monitorwhether location reporting is required, when there is a change in theserving cell, to store an indicator that the location reporting isrequired in response to a change in the serving cell, and to generate alocation report and send it to the core network 7 accordingly. Hence,the core network 7 is advantageously able to maintain is ability to keeptrack of the mobile communication device's location, when required, inresponse to the change of cell despite other S1 signalling beingterminated in the small cell gateway 14.

This exemplary embodiment also provides greater flexibility by providinga mechanism for supporting both direct and indirect forwarding ofdownlink data from the source cell 10-2 to the target cell 10-1, 10-3during handover in order to avoid, or at least mitigate, potential lossof downlink data. This mechanism involves the small cell gateway 14receiving an indication, from the source cell base station 5-2, as towhether direct data forwarding is supported or not and then providing acorresponding indication to the target cell base station 5-1, 5-3. Inthe case of direct forwarding, the downlink data can then be forwardeddirectly from the source cell base station 5-2 to the target cell basestation 5-1, 5-3 without further involvement of the small cell gateway14 in the data forwarding process. In the case of indirect forwarding,the small cell gateway 14 manages the setup and configuration of acommunication tunnel for forwarding the downlink data from the sourcecell base station 5-2 to the target cell base station 5-1, 5-2.

It will be appreciated that although this exemplary embodiment has beenintroduced with reference to S1 interface based handover a similarapproach can also be applied for signalling, during an X2 basedhandover, directed towards the core network (e.g. the MME/Se-GW). Onedifference, compared to the S1 based handover will, of course, be thepresence of X2-GW assisting in X2 signalling between small cells inpursuance of the handover.

Beneficially, in another exemplary embodiment (as illustrated in FIG.2), instead of terminating S1 signalling in the small cell gateway 14 inthe manner introduced above, the small cell gateway is operable toprovide ‘virtual’ dual connectivity in which the mobile communicationdevice 3 can have simultaneous ‘connectivity’ to two cells: a secondarycell (SCell) which, in this exemplary embodiment, is the small cell 10-2operated by the small cell base station 5-2 via which the mobilecommunication device 3 is connected to the network; and a ‘virtual’primary cell (PCell) 15 which appears, to the mobile communicationdevice 3, to be a ‘virtual macro cell’ via which the small cell isconnected to the network.

Specifically, the ‘virtual’ dual connectivity is implemented by storinga ‘virtual macro cell’ context in the small cell gateway 14. The mobilecommunication device 3 is, effectively, provided with a ‘virtual’association to the ‘virtual macro cell’ 15 but does not have a physicalassociation.

Accordingly, since the source small cell 10-2 is in the effectivecoverage of the ‘virtual’ macro cell 15, a dual connection can be usedfor handover in order to simplify the inter-node signalling exchange.Thus, when small cell to small cell handover occurs this is dealt withat the small cell gateway 14 and no signalling is required towards thecore network 7.

Mobile Communication Device

FIG. 3 is a block diagram illustrating the main components of the mobilecommunication device 3 shown in FIGS. 1 and 2. As shown, the mobilecommunication device 3 includes a transceiver circuit 310 which isoperable to transmit signals to, and to receive signals from, the basestation 5 and/or other mobile communication devices (not illustrated inFIG. 1) via at least one antenna 312. The mobile communication device 3may of course have all the usual functionalities of a conventionalmobile communication device (such as a user interface 314 which maycomprise input and output devices such as microphones, touch screen,keypad, speaker and/or the like) and this may be provided by anycombination of hardware, software and firmware, as appropriate. Theoperation of the transceiver circuit 310 is controlled by a controller316 in accordance with software stored in memory 318. The softwareincludes, among other things, an operating system 318, a communicationcontrol module 322 and a measurement module 325.

The communication control module 322 is operable to handle (e.g.generate, send and receive) control signals for controlling theconnections between the mobile communication device 3 and other userequipment or various network nodes, such as the serving base station5-2.

The measurement module 325 is operable to perform desired signalmeasurements (e.g. CRS or CSI-RS measurements), to determine associatedsignal quality values (e.g. RSRP and RSRQ) and to generate and sendassociated measurement reports as appropriate under the control of thecommunication control module 322.

Base Station

FIG. 4 is a block diagram illustrating the main components of one of thesmall cell base stations 5 shown in FIGS. 1 and 2, such as serving basestation 5-2. As shown, the base station 5 includes a transceiver circuit410 which is operable to transmit signals to, and to receive signalsfrom, the mobile communication device 3 via at least one antenna 412.The base station 5 is also operable to transmit signals to and toreceive signals from nodes in the core network 7 (such as the MME 12 orthe S-GW 16), via the small cell gateway 14, using network interface 414which comprises the S1 interface. The base station 5 is also operable totransmit signals to and to receive signals from other base stations(macro or small) either using the functionality of the small cellgateway 14 or via a dedicated eNB (X2) interface 415. The operation ofthe transceiver circuit 410 is controlled by a controller 416 inaccordance with software stored in memory 418. The software includes,among other things, an operating system 420, a communication controlmodule 422, a measurement configuration module 423, and a handovermanagement module 425 comprising a handover signalling module 426 and adata forwarding module 427.

The communication control module 422 is operable to controlcommunications between the base station 5 and the mobile communicationdevice 3, between the base station 5 and the network devices such as theMME 12, SGW 16 via the small cell gateway 14, and between the basestation 5 and other base stations via the X2 interface 415 or via theX2-GW functionality of the small cell gateway 14.

The measurement configuration module 423 is operable to configure themobile communication device 3 to perform required signal measurements(for example CRS or CSI-RS measurements) and to sending associatedinformation in a measurement report as appropriate (e.g. in response toa specific event).

The handover management module 425 is operable to manage aspects ofhandover procedures performed at the base station 5 including generationand transmission of appropriate handover signalling by the handoversignalling module 426 and managing the forwarding of downlink data viathe data forwarding module 427.

Small Cell Gateway

FIG. 5 is a block diagram illustrating the main components of the smallcell gateway 14 shown in FIGS. 1 and 2. As shown, the small cell gateway14 includes a transceiver circuit 510 which is operable to transmitsignals to, and to receive signals from, nodes in the core network 7(such as the MME 12 or the SGW 16), via an associated network interface514 comprising the S1 interface. The small cell gateway 14 is alsooperable to transmit signals to and to receive signals from basestations 5 via an eNB interface 515 including both an S1 and X2interface. The operation of the transceiver circuit 510 is controlled bya controller 516 in accordance with software stored in memory 518. Thesoftware includes, among other things, an operating system 520, acommunication control module 522, a handover management module 525comprising a handover signalling module 526 and a data forwarding module527, an S1AP termination module 528, an HeNB-GW module 529 and X2-GWmodule 530.

The communication control module 522 is operable to controlcommunications between the small cell gateway 14 and the base stationsvia the eNB interface 515 and between the small cell gateway 14 andnetwork devices such as the MME 12 and SGW 16 via the network interface514.

The handover management module 525 is operable to manage aspects ofhandover procedures performed at the small cell gateway 14 includinggeneration and transmission of appropriate handover signalling by thehandover signalling module 526 and managing the forwarding of downlinkdata via the data forwarding management module 527.

The S1AP termination module 528 provides S1AP functionality at the smallcell gateway thereby allowing the small cell gateway to terminate S1signalling at the small cell gateway (e.g. for implementing theembodiment in which S1 signalling is terminated at the gateway), and inparticular handover related signalling. The S1AP functionality includesthe ability to process S1 messages received from the base station 5 andto generate appropriate S1 response messages without the involvement ofthe core network. It will be appreciated that although this module maynot be necessary for the exemplary embodiment of FIG. 5 it may,nevertheless, be present to allow the flexibility to terminate ofsignalling if required.

The HeNB-GW module 529 provides the HeNB-GW functionality to allow thesmall cell gateway to continue to act as an intermediary between thebase station 5 and the core network 7 (e.g. for S1 signalling that isnot terminated at the small cell gateway 14 such as the exemplaryembodiment in which S1 signalling is not terminated at the gateway) andthe X2-GW module 530 provides the X2-GW functionality to allow the smallcell gateway to act as an intermediary between the base station 5 andother base stations for X2 based signalling.

In the above description, the mobile communication device 3, the basestation 5 and the small cell gateway 14 are each described for ease ofunderstanding as having a number of discrete modules (such as thecommunications control modules, the reporting module, and the handovercommand module). Whilst these modules may be provided in this way forcertain applications, for example where an existing system has beenmodified to implement the invention, in other applications, for examplein systems designed with the inventive features in mind from the outset,these modules may be built into the overall operating system or code andso these modules may not be discernible as discrete entities. Thesemodules may also be implemented in software, hardware, firmware or a mixof these.

Operation—Normal Handover

FIG. 6 illustrates operation of the exemplary embodiment in which S1handover related signalling is terminated at the gateway during a normalhandover from one small cell to another.

As seen in FIG. 6, when a handover becomes necessary (at S600), thesource small cell base station 5-2 generates and sends an S1 messagetowards the core network 7 (i.e. to the small cell gateway 14) (at S612)to indicate that handover is required and to request that handover takesplace (e.g. an S1AP: Handover Required message).

The small cell gateway 14 intercepts the message indicating thathandover is required and does not forward it to the MME 12 but insteadprocesses it in the small cell gateway 14 and generates a messagerequesting handover (e.g. an S1AP: Handover Request message) and sendsit (at S614) to the base station 5-1, 5-3 of the target cell 10-1, 10-3and informing the base station 5-1, 5-3 of the target cell 10-1, 10-3 ofrelevant information such as a list providing details of each RadioAccess Bearer (E-RAB) that needs to be set up.

In order to allow the small cell gateway 14 to provide the relevantinformation to encode the S1AP Handover Request message that are sent tosmall cell base station directly, the relevant information (UE context)is stored at the small cell gateway 14 (e.g. information identifying theE-RABs that have been established, transport layer address, E-RAB levelquality of service parameters, Aggregate Maximum Bit Rate) inassociation with information identifying the mobile communication deviceto which the information relates. This information is obtained byintercepting appropriate S1 messages that are forwarded to and from theMME 12 during normal operation (e.g. operation to set up communication,establish radio access bearers etc.) and storing relevant messagecontents for later use while preparing messages, like the S1: HandoverRequest, for facilitating small gateway terminated handover procedures.It will be appreciated that if the small cell gateway 14 is unable tofill in all the relevant information it can revert to forwarding the S1messages to the MME 12 for the MME 12 to handle the handover signallingappropriately.

The base station 5-1, 5-3 of the target cell 10-1, 10-3 responds (atS616) with a suitable acknowledgement (e.g. an S1AP: Handover RequestAcknowledgement message) which may indicate that the appropriatecommunication bearers have been successfully set up.

The small cell gateway 14 intercepts the acknowledgement message anddoes not forward it to the MME 12 but instead processes it in the smallcell gateway 14 and generates and sends a message instructing handover(e.g. an S1AP: Handover Command message) to the base station 5-2 of thesource cell 5-2 (at S628).

At this point forwarding of undelivered downlink data takes place (ifpossible and if required) between the base station 5-2 of the sourcecell 10-2 and the base station 5-1, 5-3 of the target cell 10-1, 10-3(at S630).

The target small cell base station 5-1, 5-3 sends an appropriatenotification message (e.g. an S1AP: Handover Notify message) (at S632),once the mobile communication device 3 subject to the handover has beenidentified in the target cell 10-1, 10-3 and that S1 handover has beencompleted at the target end.

The small cell gateway 14 intercepts the notification message and doesnot forward it to the MME 12 but instead processes it in the small cellgateway 14 and generates and sends a message instructing release of themobile communication device's context at the source cell 10-2 (e.g. anS1AP: UE Context Release Command message) to the base station 5-2 of thesource cell 5-2 (at S636).

The base station 5-2 of the source cell 10-2 responds by releasing themobile communication device's context and confirms that the mobilecommunication device's context has been released at the source cell 10-2by sending an appropriate message (e.g. an S1AP: UE Context ReleaseComplete message) to the small cell gateway 14 (at S638).

It will be appreciated that in the above operation, the security contextis not updated between MME 12 and small cell gateway 14 after the mobiledevice has moved from one small cell to another small cell because thereis no signalling towards the core network 7. In order to mitigate thesecurity risk that this might cause, a number of handovers or ‘hops’ canbeneficially be defined (e.g. by the operator) during which handoversignalling will terminate in the small cell gateway. Once thispredefined number of ‘hops’ has been reached the next small cell gateway14 routes handover signalling via the core network for the next handoverin order to ensure that the security context is updated. The small cellgateway 14 can return to small cell gateway terminated signalling foranother set of ‘hops’ and so on. Accordingly, in this way security iscompromised for a configurable number of hops only. Informationidentifying the predetermined number of hops can be exchanged betweenMME 12 and small cell gateway 14 using S1 signalling or can be providedby an operation and maintenance (OAM) function.

Operation—Partial Rejection

FIG. 7 illustrates operation of the exemplary embodiment in which S1handover related signalling is terminated at the gateway during ahandover from one small cell to another in which handover is partiallyrejected, e.g. due to a failure in the set up of some radio accessbearers at the base station 5-1, 5-3 of the target cell 10-1, 10-3.

As seen in FIG. 7, when a handover becomes necessary (at S700), thesource small cell base station 5-2 proceeds much in the same way as inFIG. 6 by generating and sending an S1 message to the small cell gateway14 (at S712) to indicate that handover is required and to request thathandover takes place (e.g. an S1AP: Handover Required message).

The small cell gateway 14 intercepts the message indicating thathandover is required and does not forward it to the MME 12 but insteadprocesses it in the small cell gateway 14 and generates a messagerequesting handover (e.g. an S1AP: Handover Request message) and sendsit (at S714) to the base station 5-1, 5-3 of the target cell 10-1, 10-3and informing the base station 5-1, 5-3 of the target cell 10-1, 10-3 ofrelevant information such as a list providing details of each RadioAccess Bearer (E-RAB) that needs to be set up.

The base station 5-1, 5-3 of the target cell 10-1, 10-3 responds (atS716) with a suitable acknowledgement (e.g. an S1AP: Handover RequestAcknowledgement message) which indicates that the some of thecommunication bearers have not been successfully set up (e.g. in anE-RABs Failed to Setup List information element).

The small cell gateway 14 intercepts the acknowledgement message anddoes not forward it to the MME 12 but instead processes it in the smallcell gateway 14 and determines that handover has, in effect, beenpartially rejected. The small cell gateway 14 responds to thisdetermination (at S718) by sending a message to the MME 12 identifyingthe bearers that have failed to set up (e.g. in a E-RABs Released Listinformation element (IE) in an S1AP: E-RAB Release Indication message).

The MME 12 may respond by determining a new aggregate maximum bit rateand by notifying the small cell gateway 14 (at S720) of the newaggregate maximum bit rate in an appropriate message (e.g. S1AP: UEContext Modification Request with a UE Aggregate Maximum Bit Rate IE setappropriately). The small cell gateway 14 forwards the message carryingthe new aggregate maximum bit rate to the base station 5-1, 5-3 of thetarget cell 10-1, 10-3 (at S722).

The base station 5-1, 5-3 of the target cell 10-1, 10-3 responds to themessage carrying the new aggregate maximum bit rate by modifying thecontext of the mobile communication device 3 in the target cell 10-1,10-3 by changing the aggregate maximum bit rate appropriately and bygenerating and sending a corresponding response message (e.g. an S1AP:UE Context Modification Response message) to the small cell gateway 14(at S724).

The small cell gateway 14 also generates and sends a message instructinghandover (e.g. an S1AP: Handover Command message) to the base station5-2 of the source cell 5-2 (at S728). At this point forwarding ofundelivered downlink data takes place (if possible and if required)between the base station 5-2 of the source cell 10-2 and the basestation 5-1, 5-3 of the target cell 10-1, 10-3 (at S730).

The target small cell base station 5-1, 5-3 sends an appropriatenotification message (e.g. an S1AP: Handover Notify message) (at S732),once the mobile communication device 3 subject to the handover has beenidentified in the target cell 10-1, 10-3 and that S1 handover has beencompleted at the target end.

The small cell gateway 14 intercepts the notification message and doesnot forward it to the MME 12 but instead processes it in the small cellgateway 14 and generates and sends a message instructing release of themobile communication device's context at the source cell 10-2 (e.g. anS1AP: UE Context Release Command message) to the base station 5-2 of thesource cell 5-2 (at S736).

The base station 5-2 of the source cell 10-2 responds by releasing themobile communication device's context and confirms that the mobilecommunication device's context has been released at the source cell 10-2by sending an appropriate message (e.g. an S1AP: UE Context ReleaseComplete message) to the small cell gateway 14 (at S738).

Operation—Location Reporting

FIG. 8 illustrates operation of the exemplary embodiment in which S1handover related signalling is terminated at the gateway during a normalhandover from one small cell to another but in which the MME 12 requireslocation information.

As seen in FIG. 8, before handover becomes necessary, the MME 12indicates (at S802) that location reporting is required by sending anappropriate location reporting control message indicating that locationreporting is required in the event of a change of serving cell) (e.g. anS1AP Location Reporting Control message with a Request Type IE set toEvent IE in the Request Type IE set to “Change of service cell”). Itwill be appreciated that an inbound mobility request (e.g. an S1AP:Handover Request message) from MME 12 could be similarly configured toindicate that location reporting is required.

The small cell gateway 14 stores an indication that location reportingis required in the event of a change of serving cell at S804.

When a handover becomes necessary (at S800), the source small cell basestation 5-2 proceeds much in the same way as in FIG. 6 by generating andsending an S1 message to the small cell gateway 14 (at S812) to indicatethat handover is required and to request that handover takes place (e.g.an S1AP: Handover Required message).

The small cell gateway 14 intercepts the message indicating thathandover is required and does not forward it to the MME 12 but insteadprocesses it in the small cell gateway 14 and generates a messagerequesting handover (e.g. an S1AP: Handover Request message) and sendsit (at S814) to the base station 5-1, 5-3 of the target cell 10-1, 10-3and informing the base station 5-1, 5-3 of the target cell 10-1, 10-3 ofrelevant information such as a list providing details of each RadioAccess Bearer (E-RAB) that needs to be set up.

The base station 5-1, 5-3 of the target cell 10-1, 10-3 responds (atS816) with a suitable acknowledgement (e.g. an S1AP: Handover RequestAcknowledgement message) which may indicate that the appropriatecommunication bearers have been successfully set up.

The small cell gateway 14 intercepts the acknowledgement message anddoes not forward it to the MME 12 but instead processes it in the smallcell gateway 14 and generates and sends a message instructing handover(e.g. an S1AP: Handover Command message) to the base station 5-2 of thesource cell 5-2 (at S828).

At this point forwarding of undelivered downlink data takes place (ifpossible and if required) between the base station 5-2 of the sourcecell 10-2 and the base station 5-1, 5-3 of the target cell 10-1, 10-3(at S830).

The target small cell base station 5-1, 5-3 sends an appropriatenotification message (e.g. an S1AP: Handover Notify message) (at S832),once the mobile communication device 3 subject to the handover has beenidentified in the target cell 10-1, 10-3 and that S1 handover has beencompleted at the target end.

The small cell gateway 14 intercepts the notification message and doesnot forward it to the MME 12 but instead processes it in the small cellgateway 14 and determines that location reporting is now required as aresult of the change of cell. Accordingly, the small cell gateway 14generates and sends a location report (e.g. an S1AP: LOCATION REPORTmessage) as S834 to the MME 12 in accordance with the earlier requestand associated stored information.

The small cell gateway 14 then generates and sends a message instructingrelease of the mobile communication device's context at the source cell10-2 (e.g. an S1AP: UE Context Release Command message) to the basestation 5-2 of the source cell 5-2 (at S836).

The base station 5-2 of the source cell 10-2 responds by releasing themobile communication device's context and confirms that the mobilecommunication device's context has been released at the source cell 10-2by sending an appropriate message (e.g. an S1AP: UE Context ReleaseComplete message) to the small cell gateway 14 (at S838).

Operation—Status Transfer

FIG. 9 illustrates operation of the exemplary embodiment in which S1handover related signalling is terminated at the gateway during a normalhandover from one small cell to another but in which a transfer ofstatus information (e.g. uplink/downlink Packet Data ConvergenceProtocol (PDCP) Sequence Number (SN) and/or hyper frame number (HFN)) isrequired in support of in-sequence delivery of data and duplicationavoidance for the handover. Such a status transfer may be required, forexample, for each respective E-RAB for which PDCP-SN and HFN statuspreservation applies.

As seen in FIG. 9, when a handover becomes necessary (at S900), thesource small cell base station 5-2 proceeds much in the same way as inFIG. 6 wherein the steps labelled using the format S6nn in FIG. 6correspond to steps sharing the same last two digits but labelled usingthe format S9nn in FIG. 9.

Unlike FIG. 6, however, in FIG. 9 the base station 5-2 of the sourcecell 10-2 determines, after any data forwarding at S930, that a transferof status information (e.g. uplink/downlink Packet Data ConvergenceProtocol (PDCP) Sequence Number (SN) and/or hyper frame number (HFN)) isrequired in support of in-sequence delivery of data and duplicationavoidance for the handover. The base station 5-2 of the source cell 10-2generates and sends an appropriate status transfer message (e.g. anS1AP: eNB Status Transfer Message) towards the core network 7 at S920.

The small cell gateway 14 intercepts the status transfer message anddoes not forward it to the MME 12 but instead processes it in the smallcell gateway 14 and in response to the message generates an appropriatestatus transfer message (e.g. an S1AP: MME Status Transfer Message) forsending to the base station 5-1, 5-3 of the target cell 10-1, 10-3.

The remaining procedure then continues much in the same way as in FIG.6.

Operation—Data Forwarding

There are two data forwarding schemes which are supported by the smallcell gateway 14 for S1 based Small Cell to Small Cell handover. The twoschemes include direct data forwarding and indirect data forwarding andthese are described below.

Moreover, in the case of indirect data forwarding, two different methodsare described below depending on the architecture chosen for the smallcell gateway 14. The two different architectures are illustrated inFIGS. 10 and 11.

In the architectures shown in FIGS. 10 and 11 the LTE Femto cellscorrespond to the small cells 10 described above and these are connectedto small cell gateway apparatus 14 (in this example a HeNB gatewaysystem). The HeNB gateway system in each case comprises security gateway(SeGW) to which the small cells are connected into the small cellgateway apparatus 14 via the internet and an S1 interface. The SeGW isconnected via an IP router to a home location register (HLR) via anAuthentication, Authorization and Accounting (AAA) function, toOperation and Maintenance (OA&M) network, and to a HeNB GW function. Ineach of the architectures shown in FIGS. 10 and 11 control planesignalling is routed, from the IP router, to the MME 12 of the corenetwork 7 via an S1-MME interface and the HeNB GW function. In thearchitecture shown in FIG. 11 user plane signalling is also routed, fromthe IP router, to the core network 7 (to the SGW 16) via the HeNB GWfunction (and an S1-U interface). In the architecture shown in FIG. 10,however, user plane signalling is not routed, from the IP router, viathe HeNB GW function but is instead routed directly, from the IP router,via an S1-U interface to the SGW 16 of the core network 7. The SeGW maybe a physically separate communication entity to the HeNB GW.

Accordingly, in the architecture of FIG. 10, the user plane isterminated in the SeGW and the control plane is terminated in the HeNBGW whereas, in the architecture of FIG. 11, the user and control planesare each terminated in the HeNB GW.

Similarly, in the architecture of FIG. 10, GPRS Tunnelling Protocol(GTP) functions are provided in the SeGW whereas, in the architecture ofFIG. 11, these are provided in the HeNB GW.

Direct Data Forwarding

If the direct forwarding is available from the source small cell 10-2 tothe target small cell 10-1, 10-3, then the base station 5-2 of thesource small cell 10-2 indicates this in the S1 message sent to thesmall cell gateway 14 to indicate that handover is required and torequest that handover takes place (e.g. by setting a Direct ForwardingPath Availability IE to “Direct Path is Available” in the S1AP: HandoverRequired message sent at S612 in FIG. 6 or corresponding steps in FIGS.7 to 9).

The small cell gateway 14 processes the message indicating that handoveris required, determines that direct forwarding is available, and henceinforms the base station 5-1, 5-3 of the target small cell 10-1, 10-3 ofthis in the message requesting handover (e.g. in the S1AP: HandoverRequest message sent at S614 in FIG. 6 or corresponding steps in FIGS. 7to 9). Specifically, the small cell gateway 14 indicates that directforwarding is available by deliberately not configuring, in the S1AP:Handover Request message, the “Data forwarding not possible IE” for eachbearer represented in the “E-RABs To Be Setup Item IEs” for which directforwarding is available.

When the small cell gateway 14 sends the message instructing handover(e.g. an S1AP: Handover Command message) to the base station 5-2 of thesource cell 5-2 (e.g. at S628 in FIG. 6 or corresponding steps in FIGS.7 to 9). The small cell gateway 14 configures the message instructinghandover to identify the addresses allocated, and associated tunnellingidentities (TEIDs), for each radio access bearer subject to forwarding,for example by configuring the “E-RABs Subject to Forwarding List” IE inthe S1AP: Handover Command message to include a list of addresses andTEIDs allocated for forwarding. The E-RABs to Release List IE include alist of all the bearers to be released.

After this, the base station 5-2 of the source cell 5-2 can startforwarding downlink data towards the base station 5-1, 5-3 of the targetsmall cell 10-1, 10-3 (e.g. at S630 in FIG. 6 or corresponding steps inFIGS. 7 to 9). During this data forwarding process, therefore, the basestation 5-2 of the source cell 5-2 forwards downlink data directly tothe base station 5-1, 5-3 of the target small cell 10-1, 10-3. The SeGWin the case of the variant shown in FIG. 10 and the HeNB-GW function inthe case of the variant shown in FIG. 11 is not involved.

Indirect Data Forwarding

If the direct forwarding is not available from the source small cell10-2 to the target small cell 10-1, 10-3, then the base station 5-2 ofthe source small cell 10-2 indicates this in the S1 message sent to thesmall cell gateway 14 to indicate that handover is required and torequest that handover takes place (e.g. by setting a Direct ForwardingPath Availability IE to “Direct Path is Not Available” in the S1AP:Handover Required message sent at S612 in FIG. 6 or corresponding stepsin FIGS. 7 to 9).

The small cell gateway 14 processes the message indicating that handoveris required, determines that direct forwarding is not available, andhence informs the base station 5-1, 5-3 of the target small cell 10-1,10-3 of this in the message requesting handover (e.g. in the S1AP:Handover Request message sent at S614 in FIG. 6 or corresponding stepsin FIGS. 7 to 9). Specifically, the small cell gateway 14 indicates thatdirect forwarding is not available by deliberately configuring, in theS1AP: Handover Request message, the “Data forwarding not possible IE”for each bearer represented in the “E-RABs To Be Setup Item IEs” forwhich direct forwarding is not available.

In the case of the architecture in FIG. 10, the HeNB-GW uses parameters(target Small Cell addresses, TED and E-RAB pair for forwarding) forsetting up the forwarding path tunnel by sending a forwarding tunnelcreation request (e.g. a Create Indirect Data Forwarding Tunnel Requestmessage) to SeGW to initiate configuration of an appropriatecommunication tunnel for forwarding the data. The SeGW responds with acorresponding response message (e.g. a Create Indirect Data ForwardingTunnel Response message) comprising SeGW addresses, TED and informationidentifying the E-RAB pair for forwarding.

In the case of the architecture in FIG. 11 the HeNB-GW function usesappropriate parameters (target Small Cell addresses and TEIDs forforwarding) for setting up the forwarding path tunnel.

When the small cell gateway 14 sends the message instructing handover(e.g. an S1AP: Handover Command message) to the base station 5-2 of thesource cell 5-2 (e.g. at S628 in FIG. 6 or corresponding steps in FIGS.7 to 9). The small cell gateway 14 configures the message instructinghandover to identify the addresses allocated, and associated tunnellingidentities (TEIDs), for each radio access bearer subject to forwarding,for example by configuring the “E-RABs Subject to Forwarding List” IE inthe S1AP: Handover Command message to include a list of addresses andTEIDs allocated for forwarding. The E-RABs to Release List IE include alist of all the bearers to be released.

After this, the base station 5-2 of the source cell 5-2 can startforwarding downlink data towards the base station 5-1, 5-3 of the targetsmall cell 10-1, 10-3 (e.g. at S630 in FIG. 6 or corresponding steps inFIGS. 7 to 9). During this data forwarding process, therefore, the basestation 5-2 of the source cell 5-2 forwards downlink data indirectly tothe base station 5-1, 5-3 of the target small cell 10-1, 10-3 via theforwarding path tunnel.

In the case of the architecture in FIG. 10, during the data forwardingprocess, the SeGW effectively works as the U-plane anchor point forforwarding the U-plane packets from the source small cell 10-2 to thetarget small cell 10-1, 10-3.

In the case of the architecture in FIG. 11, during the data forwardingprocess, the HeNB-GW effectively works as the U-plane anchor point toforward the U-plane packets from the source small cell 10-2 to thetarget small cell 10-1, 10-3.

On receipt of the notification message (e.g. an S1AP: Handover Notifymessage) informing the small cell gateway that handover has effectivelybeen completed at the target end (e.g. at S632 in FIG. 6 orcorresponding steps in FIGS. 7 to 9), the HeNB-GW starts a timer forreleasing the data forwarding resource.

In the case of the architecture in FIG. 10, after the timer has expiredthe HeNB-GW generates a message to initiate release of the temporaryresources used for indirect forwarding (e.g. a Delete Indirect DataForwarding Tunnel Request message) and sends it to the SeGW to initiatethe release.

In the case of the architecture in FIG. 11, after the timer expires, theHeNB-GW releases the temporary resources used for indirect forwardingitself.

Dual Connectivity

As explained above FIG. 2 illustrates an embodiment in which a ‘virtual’dual connectivity is advantageously provided even though there is nophysical macro cell in the vicinity. It will be appreciated thatalthough this is described as a different embodiment some (or all) ofthe features of both embodiments could, potentially, be implemented inthe same communication device as appropriate in order to providedifferent options to an end user.

FIGS. 12 and 13 respectively show a control plane architecture and auser plane architecture in a communication system in which ‘virtual’dual connectivity is provided by having the small cell gateway 14administer a virtual macro cell which acts to terminate X2 signalling.

In order to achieve ‘virtual’ dual connectivity the ‘virtual’ macro cellcontext is stored in the small cell gateway 14, and the mobilecommunication device 3 has a virtual association to the virtual macrocell 15 but not a physical association. This virtual macro cell 15 actsas a primary cell (PCell) whilst the small cells 10 operate as secondarycells (SCells) in which control signalling is limited.

As seen in FIGS. 12 and 13, in the control plane, the mobilecommunication device 3 has a radio resource control (RRC) and othercontrol connections to the small cell base station 5 as normal. No RRCstate machine is provided in the small cell gateway 14 but the gatewaymaintains the RRC context for the mobile communication device 3. Smallcells may be added/deleted as secondary cells in a manner similar toScell addition/deletion as defined, for example in section 5.3.10.3b ofTS 36.331 V11.3.0.

The mobile communication device 3 maintains security keys derived fromthe initial connection with the first small cell 10 operating as asecondary cell (SCell) to the virtual macro cell 15 acting as the PCell.The mobile communication device 3 is configured to store and maintainthese keys in response to an indication that these keys should bepreserved provided by the base station 5 operating the first cell viawhich the mobile communication device 3 connects (e.g. following initialconfiguration of the non access stratum (NAS) and access stratum (AS)security contexts).

More specifically, the mobile communication device 3 is explicitlyinformed that the virtual macro cell 15 exists and stores security keys(AS and NAS) accordingly. During a move from one small cell to another,the source small cell base station 5 transfers the security keys tosmall cell gateway 14 as part of the security context for that mobilecommunication device and the gateway continues to transfer these keys tothe respective base station 5 of each small cell 10 to which the mobilecommunication device 3 moves. The number of times that the security keysare transferred in this way is limited to a predetermined number of‘hops’ which may be reconfigurable or fixed. It will be appreciated thatwhere all the small cells in one hop use the same tracking area identitythere is no issue related to NAS mobility information reference.

When the number of ‘hops’ reaches its limit, normal security handlingshall be performed to re-initialise the security context with new keysthereby limiting any security risk associated with the re-use of keysand the transfer of keys between small cell base station 5 and the smallcell gateway 14.

Unlike existing methods for adding Scells, the addition of SCells forthe purposes of virtual dual connectivity involves the setting up of aPacket Data Convergence Protocol (PDCP), Radio Link Control (RLC) andMedium Access Control (MAC) configuration during adding another smallcell 10 as SCell by including PDCP, RLC and MAC information in the RRCmessage.

In order to alleviate the additional signalling overhead associated withthe need to setting up the PDCP, RLC and MAC configurations the ‘delta’signalling may be used to signal only modifications to theconfigurations (e.g. only new/modified parameters compared to anexisting configuration).

The mobile communication device 3 continues to receive systeminformation in connected mode from the base station 5 of the small cell10 via which it is connected. The virtual Pcell 15 does not send anysystem information because no system information exists for such a Pcell15. Similarly, Earthquake and Tsunami Warning System (ETWS)/CommercialMobile Alert System (CMAS) notifications are provided from via the smallcell 10 acting as an Scell only because no such information is availablefrom the virtual Pcell 15. The mobile communication device 3 alsoreceives paging information from the base station 5 of the small cell10.

Unlike current systems in which a Pcell Cell Radio Network TemporaryIdentifier (C-RNTI—an identifier allocated by a base station, to amobile communication device, to be unique within one cell controlled bythat base station) is used in SCells as well. The C-RNTI for an SCell toa virtual PCell is carried out by the base station 5 of the small cell10. Whilst the same C-RNTI allocation could be kept throughout multiple‘hops’ from one small cell to another this may restrict the number ofavailable C-RNTIs and could potentially cause C-RNTI collision.Accordingly, in this embodiment, the C-RNTI is not kept when a move to anew small cell 10 occurs.

Operation—Dual Connectivity

Typical operation in a communication system configured for virtual dualconnectivity will now be described, by way of example only, withreference to FIGS. 14 and 15 in which FIG. 14 is a timing diagram for aninitial RRC connection establishment by a mobile communication device 3in a small cell 10 and FIG. 15 is a timing diagram illustrating ahandover scenario in a communication system configured for virtual dualconnectivity.

Referring to FIG. 14 an initial RRC connection setup procedure begins atS1400. The mobile communication device 3 generates and sends an initialUE message to the base station 5-2 of a small cell 10-2. The initial UEmessage is propagated to the MME 12 via the small cell gateway 14 usingS1 signalling (at S1402 and S1403). The MME 12 sets up of an NASsecurity context for the mobile communication device 3 at S1405 beforeinitiating setup of an AS context for the mobile communication device 3by sending an initial UE context setup message to the small cell gatewayat S1407 which is then forwarded to the base station 5-2 of the smallcell 10-2 at S1408. A reconfiguration message to configure bearerestablishment and inform the mobile communication device that thecontrol and user planes are virtually split between the small cell 10-2and the virtual macro cell 15. Once reconfiguration is complete themobile communication device confirms this to the base station 5-2 of thesmall cell 10-2 at S1411 and the base station 5-2 of the small cell 10-2responds by sending an initial context setup response message to thesmall cell gateway 14 at S1412. The small cell gateway 14 forwards theinitial context setup response message to the MME 12 at S1413 tocomplete set up of the initial connection. GTP communication tunnels arethus setup (S1415 and S1417) between the base station 5-2 of the smallcell 10-2 and the small cell gateway 14 and between the small cellgateway 14 and the MME 12.

Referring to FIG. 15 a measurement report is sent to the base station5-2 of a source small cell 10-2 at S1501. The base station 5-2 of thesource small cell 10-2 determines that a transfer of cell may benecessary and prepares for this by transferring the mobile communicationdevice's context (RRC, S1 and GTP) to the small cell gateway 14 at S1503(e.g. using appropriate X2/X3 signalling such as an X3-AP: ContextTransfer message) before forwarding the measurement report to the smallcell gateway 14, using appropriate X2/X3 signalling including a targetidentifier for the target cell 10-1, 10-3 at S1504 (e.g. usingappropriate X2/X3 signalling such as an X3-AP: Measurement Reportmessage). The small cell gateway 14 stores the context for the mobilecommunication device 3 and, on receipt of the measurement report,initiates setup of a corresponding context for the mobile communicationdevice 3 at the base station 5-1, 5-3 of the target small cell 10-1,10-3 (e.g. using appropriate X2/X3 signalling such as an X3-AP: ContextSetup message). The base station 5-1, 5-3 of the target small cell 10-1,10-3 responds at S1507 to indicate that the context has been set upsuccessfully. The small cell gateway 14 then responds, at S1509,appropriately to the original context transfer message from the basestation 5-2 of the source small cell 10-2. On receipt of the responsefrom the small cell gateway 14 the base station 5-2 of the source smallcell 10-2 reconfigures the mobile communication device 3 by providing,at S1511, appropriate information to allow the mobile communicationdevice 3 to connect to the base station 5-1, 5-3 of the target cell10-1, 10-3 (e.g. in an SCell INFO information element or the like). Oncethe reconfiguration has completed successfully this is indicateddirectly to the base station 5-1, 5-3 of the new (target) cell 10-1,10-3 at S1513. The base station 5-1, 5-3 of the new (target) cell 10-1,10-3 notifies the small cell gateway 14 of this at S1515 and the smallcell gateway 14 notifies the base station 5-2 of the old (source) cell10-2.

In order to support the dual connectivity, the target small cell basestation is added by using an Scell addition/deletion procedure (incontrast to the approach for a normal mobility procedure). For thisprocedure, security keys can advantageously be reused (without involvingkey derivation with the core network).

Based on the GTP context (e.g. E-RAB information of E-RAB ID, Transportlayer address and GTP-TEID) exchanged during the S1505, S1507, the GTPtunnel is set up between the target small cell base station and thesmall cell gateway at S1519. Hence, user data packets can be transferredfrom the target small cell base station to the small cell gateway andthen to the S-GW.

Referring to steps S1521 and S1523, the small cell gateway 14 isoperable to terminates S1AP messages from the MME 12 (such as the S1APBearer Modification Request shown at S1521) so that the small cellgateway 14 can update the information stored at the gateway 14 (e.g. ina database) with the latest context for the mobile communication deviceaccording to the received S1AP message. The small cell gateway 14 thenforwards the message to the correct small cell base station based on thecontext information for the mobile communication device (at S1523).

MODIFICATIONS AND ALTERNATIVES

A number of detailed exemplary embodiments have been described above. Asthose skilled in the art will appreciate, a number of modifications andalternatives can be made to the above exemplary embodiments whilst stillbenefiting from the inventions embodied therein.

Referring to the exemplary embodiment shown in FIG. 2 it will beappreciated that data forwarding could be implemented by having thesource cell base station transfer the PDCP SN and Service Data Units(SDUs) for which no acknowledgement has been received, to the small cellgateway and having the small cell gateway forward this information tothe target small cell base station. Such information can be transferredover an X3, a new interface, or an existing S1/X2 interface.

It will be appreciated that the MME may transfer the context to smallcell gateway in a new S1 message instead of having a small cell transferit as described with reference to FIG. 15. The small cell gateway maystill intercept S1 messages so that when it receives an S1 message tomodify a communication bearer the gateway knows where to route it to.

In the above exemplary embodiments, a mobile telephone basedtelecommunications system was described. As those skilled in the artwill appreciate, the signalling techniques described in the presentapplication can be employed in other communications system. Othercommunications nodes or devices may include user devices such as, forexample, personal digital assistants, laptop computers, web browsers,etc.

In the exemplary embodiments described above, the base station and thehome base station each include transceiver circuitry. Typically thiscircuitry will be formed by dedicated hardware circuits. However, insome exemplary embodiments, part of the transceiver circuitry may beimplemented as software run by the corresponding controller.

In the above exemplary embodiments, a number of software modules weredescribed. As those skilled in the art will appreciate, the softwaremodules may be provided in compiled or un-compiled form and may besupplied to the HENB or to the mobile telephone as a signal over acomputer network, or on a recording medium. Further, the functionalityperformed by part or all of this software may be performed using one ormore dedicated hardware circuits. However, the use of software modulesis preferred as it facilitates the updating of the base stations,gateways, and the mobile telephones in order to update theirfunctionalities.

Various other modifications will be apparent to those skilled in the artand will not be described in further detail here.

This application is based upon and claims the benefit of priority fromUnited Kingdom patent application No. 1306216.1, filed on Apr. 5, 2013,the disclosure of which is incorporated herein in its entirety byreference.

The invention claimed is:
 1. A method in a gateway apparatus in acommunication system comprising a plurality of base stations, the methodcomprising: communicating in a network in which a user equipment (UE)operates in a dual connectivity mode in which the UE has a radioresource control (RRC) connection and is configured with a plurality ofcells; receiving, from a first base station that provides a first cellfor said dual connectivity, via a first X2 interface, a message fortransferring, to a second base station that provides a second cell forsaid dual connectivity, context information including informationrelating to the first base station and information relating to thesecond base station, the context information comprising E-RAB contextinformation in the form of an E-RAB ID, Packet Data Convergence Protocol(PDCP) Sequence Number (SN) and hyper frame number (HFN); andtransmitting, to the second base station, via a second X2 interface, theE-RAB context information in the form of an E-RAB ID, PDCP SN and HENincluded in the message for transferring context information, whereinthe UE continues to operate in the dual connectivity mode after thecontext information is transmitted to the second base station.
 2. Agateway apparatus in a communication system comprising a plurality ofbase stations, the gateway apparatus comprising: a first X2 interfacewith a first base station; a second X2 interface with a second basestation; a transceiver configured to: communicate in a network in whicha user equipment (UE) operates in a dual connectivity mode in which theUE has a radio resource control (RRC) connection and is configured witha plurality of cells, wherein the first base station provides a firstcell for said dual connectivity and the second base station provides asecond cell for said dual connectivity; receive, from the first basestation via the first X2 interface, a message for transferring, to thesecond base station, context information including information relatingto the first base station and information relating to the second basestation, the context information comprising E-RAB context information inthe form of an E-RAB ID, Packet Data Convergence Protocol (PDCP)Sequence Number (SN) and hyper frame number (HFN); and transmit, to thesecond base station via the second X2 interface, the E-RAB context andinformation in the form of an E-RAB ID, PDCP SN and HFN included in themessage for transferring context information, wherein the UE continuesto operate in the dual connectivity mode after the context informationis transmitted to the second base station.